Travelling wave apparatus



Sept. 9, 1958 Filed Feb. 2. 1954 D. C. ROGERS TRAVELLING WAVE! APPARATUS 5 Sheets-Sheet 1 Inventor D. C. R O GERS A ttorney P 9, 1.958 I v D. c. ROGERS- 2,851,629 TRAVELLING WAVE APPARATUS Filed Feb. 2. 1954 3 Sheets-Sheet 2 I v A 24 T/34v W1 m Twirl H W. W M W W W W I Inventor D. C. ROGERS A ttorney D. C. ROGERS TRAVELLING WAVE APPARATUS Sept 9, 1958 Filed Feb. 2, 1954 3 Sheets-Sheet 3 Inventor D. C. ROG E R5 Attorney Patented Sept. 9, 1958 TRAVELLING WAVE APPARATUS Douglas Cecil Rogers, London, England, assignor to Enternational Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application February 2, 1954, Serial No. 407,731

(Ilaims priority, application Great Britain March 26, 1953 9 Claims. (Cl. 315-35) The present invention relates to travelling-wave apparatus and is particularly concerned with the construc-- tion of a circuit unit for travelling-wave tubes. By a circuit unit, often abbreviated to circuit, we mean the physical assembly of apparatus as a unitary construction, minus the travelling-Wave tube itself, comprising means for mounting the tube, a magnet assembly for provicling a magnetic focussing field for the electron beam and at least an output high frequency transmission path, for example a length of rectangular wave guide terminating in a wave guide coupling flangethe output terminal of the circuit. In the case of an amplifier, as contrasted with an oscillator, an input wave energy connection is included in the unit. In addition to the above items the circuit unit may comprise means for adjusting the impedance match between the travelling-wave tube and apparatus external to the circuit unit and, also certain ancillary features such as cooling attachments, ducts for passage of coolant fluid and the like;

In travelling-wave apparatus an electron beam is projected along the axis of a helix or other wave guide structure adapted to propagate electromagnetic waves along the said axis with a phase velocity of the same order as the axial velocity of the electrons of the beam. An interchange of energy between the electromagnetic waves and the electrons of the beam occurs. Although heretofore normally used as a high frequency amplifier, travelling-wave apparatus can, also often be arranged as an oscillator or other circuit device, which, at lower frequencies, would depend for its operation upon the amplifying or rectifying action of a thermionic valve.

According to the present invention there is provided travellin -wave apparatus comprising an assembly of solenoids arranged about a common axis to provide a magnetic field focussing the electron beam of a travellingwave tube along the said axis; at least one high frequency transmission path (e. g. a hollow waveguide) mounted between two of the said solenoids for providing electromagnetic field coupling between the said tube and an external circuit; means for mounting the said travellingwave tube within the said apparatus; a pair of centrally apertured end plates of ferromagnetic material mounted on at either end of the said assembly of solenoids and means for providing a low reluctance magnetic path of ferromagnetic material placed around the. said solenoids between the said end plates.

The invention will be described with reference to the accompanying drawings in which:

Figure 1 shows, in partial cross-section, a travellingwave circuit unit, suitable for use as a travelling-wave amplifier, and a travelling-wave tube mounted therein;

Figure 2 shows, diagrammatically, a modification of the circuit of Fig. 1;

Figures 3, 4 and 5 are diagrams which will be referred to in the description of a further embodiment of the invention shown diagrammatically in Figure 6.

Y in Figure 1 reference numeral 1 indicates a travellingwave tube having an electron gun 2 housed within an t. it)

envelope bulb 3, a helix 4 supported within a tubular extension 5 of the envelope and a collector electrode 6 which closes the end of the envelope portion 5 remote from the electron gun. The collector electrode 6 fits into a cooler assembly 7 carrying heat dissipating fins 8 and insulatingly supported at 9 in a magnet assembly end plate 10 which forms an armature and pole piece at this end of the tube. Envelope portion 3 carries at its rear end a base 12. fitted with conventional valve pins 13. The base 12 is secured to a plate i4- fixed by means of bolts and spacing collars 15 to an end plate 16 of a magnet assembly 17 which is clamped between the plate 16 and the aforementioned end plate 10 by means of tie rods 18. The magnet assembly further comprises a set of solenoids 19, 2t), 21, and 22, solenoids 2t) and 21 being spaced apart by means of a brass collar 23, while an input wave guide 24, providing a high frequency transmission path, is secured between solenoids 19 and Ztl. Wave guide 24 has an extension 25 provided with a short-circuiting piston 26 and piston actuating screw 2"], for the purpose of adjusting the impedance match between the wave guide 24- and helix 4. The travelling wave tube 1 projects through the enclosure of wave guide 24 and its extension 25 so that a choke member 28, within the tube envelope, is in line with an outer choke member 2?? which is secured to the wave guide wall opposite to the adjacent end of the helix 4. Inner choke member carries a short tube 30 secured to its front face and a tube 31!. secured to its rear, the electron beam passing through the tubes 31 and 30. The end of the helix 4 is joined to the tube 3%), which provides an antenna probe for coupling between the wave guide 24 and the helix 4. The helix 5 and inner choke member 28 are supportedon quartz rods 32 within the envelope portion 5 of the travelling wave tube, while the tube 31 is secured to an insulating member 33, carried on the electron gun 2, so as to afford independent D. C. connection means to the helix 4.

The arrangements at the output end of the helix are similar in general to those described, there being provided an output wave guide 34 and an impedance match ing section 35 adjusted by means of a piston actuating screw 35. The helix. rnatcl iri'angements are similar to those described for the input end of the helix, an outer high frequency choke member 37 being visible in Figure 1 adjacent collector electrode 6, to which, however, it is not connected.

Surrounding the electron. gun end of the tube is a cylinder 38 of ferromagnetic material secured by means of a circular flange 39 to the end plate 16 of the magnet assembly. The outer member 4% of the electron gun 2 constitutes a pole-piece of the magnet system, separated by an annular air gap from the surrounding cylinder 38. The member 40 also serves as a magnetic shield surrounding the electron gun 2. To improve the magnetic field attenuation the cylinder 38 projects considerably to the rear of the plate 16, any external leakage flux tending to be collected on this rearward extension.

in mounting the travelling-wave tube Within the apparatus, care must be taken that. any slight misalignment of the supporting means of either end of the tube does not cause undue strains which might result in breakage of the tube. The cooler assembly 7' is too heavy to permit the tube to be supported at one end only. The supporting means 9 is therefore provided, but, preferably, this supporting means should provide only a small area of contact with the tube, such as obtained, by rounding the support surface, and should not be a tight fit around the supported member. At the electron gun end of the tube the base 12 carries a circular flange 41 which seats against a shoulder 420171 the plate 14 and is locked in position by means of retaining clips 4-3.

The travelling-wave tube may be removed from the circuit by releasing the clips 43, unscrewing a nut 44 which secures the cooler assembly 7 to the collector electrode 6 and withdrawing the tube.

The wave guide 24 and its extension 25 are constructed as a single section of rectangular wave guide with central apertures in opposite wide walls receiving cylindrical bushes, of which one is designated 29a and the other forms the outer choke member 29 previously referred to. The members 29 and 29a act as central bushings for solenoid support members 45 fitted to either side of the wave guide section 24, 25. Similar arrangements are provided in connection with the output wave guide section 34, 35. The two centre solenoids 20 and 21, which are supported at one end by respective members 45 just described, are supported and aligned at their adjacent ends by means of a brass bushing 23 which is formed as a hollow cylinder having a central circular flange 46.

The solenoid 19 is supported at the electron gun end of the assembly by means of a projecting flange 48 on the lower coil former member 49 which fits into a circular recess in the end plate 16. A- similar arrangement is provided at the collector end of the assembly for the support of solenoid 22. With the circuit unit constructed as described it is substantially self-supporting, the tie rods 18 serving in this connection, merely to retain the component members in position.

The magnet assembly of the circuit unit of Fig. 1 is designed so as to preserve, as far as possible, uniformity of axial magnetic field over the whole length of the travelling-wave tube between the front edge of the cylinder 38 and the electron collector electrode. Near the ends of a simple solenoid the magnetic field tends to diverge, so introducing a radial component of magnetic force which is undesirable in travelling-wave apparatus. The provision of end plates 10 and 16 of ferromagnetic material reduces the non-uniformity of the magnetic field near the ends of the assembly; the ferromagnetic tie rods 18 form a squirrel-cage structure around the solenoids between the end plates 10 and 16 which serves, not only to reduce the external leakage flux and hence interference with other apparatus, but also to shield the travelling-wave tube from external magnetic disturbances. These rods, therefore, are given a cross-section adequate to carry the flux of the magnet system without saturation. From the magnetic point of view the squirrel-cage assembly of tie rods 18 can equally well, or even better, be replaced by a soft iron cylinder surrounding the solenoid, between the end plates, the cylinder being apertured to receive the wave guides. We have found, however, that the tie rod arrangement illustrated in Fig. 1 is magnetically adequate and is more convenient mechanically.

It is usually necessary to supply a continuous flow of air around the heat dissipating fins 8 of the cooler assembly 7. It is convenient to ensure this flow by enclosing the fins in a jacket through which air flows under pressure. Such a jacket is indicated at 50 by the dotted lines and may be fitted with a nozzle 51 for connection to an air blower.

The solenoids 19, 20, 21 and 22 are proportioned relatively to one another to provide as uniform a field as possible along the axis of the system. While it would obviously be possible to substitute a single solenoid for the pair and 21, it would, in general, be necessary to grade the windings on such a single solenoid so that there was not an undue concentration of magnetic flux in the centre. In the embodiment illustrated in Fig. 1 it has been preferred to provide two similar coils separated by member 23.

The dimensions of the cylinder 38 surrounding the electron gun of the travelling-wave tube, and its configuration with respect to the end plate 16 and electron gun, depends upon the type of magnetic focussing and electron gun employed. The embodiment illustrated in Fig. 1 is designed for use according to the electron beam focussing arrangement described and claimed in my copending application Serial No. 417,116, filed March 18, 1954. In that arrangement the electrons converge electrostatically from a concave cathode to pass into a substantially electric field free region in which the mutual repulsion between the electrons counteracts the convergence of the beam. The electrons enter the magnetic field at the exit aperture of the electron gun substantially parallel to the axis of the tube. The electrostatic portion of: the electron gun, and in particular the cathode, must, as far as is possible, be in a magnetic field free space. The cathode is therefore contained within the pole-piece 40 and a portion 52 of the cylinder 38 projects Well to the rear of the electron gun so as to provide high magnetic attenuation along the axis of the cylinder, particularly in the vicinity of the cathode of the gun.

In another system of electron beam focussing the cathode is immersed in an axial magnetic field of such strength that the individual electrons are constrained, on emission from the cathode, to follow the lines of magnetic force as opposed to rotating in helical paths about the beam axis. With such a focussing system, the general arrangement of the circuit unit may be similar to that shown in Fig. l with the exception of the cylinder 38 and the number of solenoid coils employed. In general, we find that it is preferable to use two spaced-apart coils in place of the single solenoid 19 of Fig. l and, for the sake of uniformity in coil winding, to provide three coils between the two wave guides, the arrangement being shown diagrammatically in Fig. 2, in which two coils 53 and 54 are positioned between the end plate 16 and the wave guide 24, three similar coils 55, 56 and 57 are positioned between the wave guides 24 and 34 and a single coil 58 is placed between wave guide 34 and end plate 10. The plate 14 supporting the tube, which is not shown in Fig. 2, is positioned with respect to the end plate 16 so that the cathode is at least a distance equal to the radius of the pole-piece aperture 59 in front thereof. A cylinder corresponding to cylinder 38 in Fig. 1 may be omitted entirely and must not in any case surround the cathode, but a rcarwardly projecting cylinder 60, corresponding to portion 52 of Fig. 1, may be retained as a collector for any external leakage fiux which might otherwise reduce the field about the electron gun.

In the arrangement shown in Fig. 1 the several solenoids are not necessarily identical and the design calculations for the coil system are somewhat tedious. It is obviously preferable from the manufacturing point of view to reduce as far as possible the number of differently sized coils thus reducing the number of difierent types of coil which must be made. In further embodiments of the invention, this may be done, while, at the same time, simplifying the design procedure.

Let it be assumed that an infinite number of similar single-turn magnetising coils are placed about an axis as illustrated diagrammatically for a portion of the system in Fig. 3. Single-turn coils 61 are depicted in crosssection, placed side-by-side about their common axis 62, each coil being spaced distance P from its neighbours. The single-turn coils have a mean diameter D If these coils all carry the same D. C. current and the field along the axis 62 is plotted, a curve similar to that shown in Fig. 4 is obtained, in which the abscissa z are distances from an arbitrary point in the system along the axis 62 and the ordinates H give the field on the axis. Then, in general, the strength of the field will fluctuate in an ap- I proximately sinusoidal manner, as shown by the curve 63 of Fig. 4, the field varying along the axis between a maximum value H and a minimum value H It will be evident that the field fluctuation along the axis can be increased or decreased to a considerable extent by variation of the parameters P and D In Fig. 5 the curve 64 indicates very roughly the manner in which the quantity (1-H /H varies with the ratio P/D The curve is asymptotic to the axis of P/D near the origin,

but for values of P/D greater than above 0.7 the value of the ordinates increases rapidly, a knee of the curve occurring for a value of P/D between 0.5 and 0.7 at which 1-minus I-I /I-I is of the order of 0.05. It will be seen therefore that the variation of axial field is quite small for values of P/D less than 0.6.

If, now, in place of the single-turn coils 61 of Fig. 3 we substitute individual multi'turn coils, curves similar to that of the curve 64 of Fig. 5 can be plotted. It is found that, in general, their shape for different shapes of coil winding are very similar to curve 64, having a knee in much the same position and tending to be asymptotic to the P/D m axis if produced towards the origin. Since, of course, with two coils of finite size the distance P between their central planes cannot be reduced to zero, the curves for practical coils terminate before the origin is reached.

Fig. 6 shows diagrammatically, omitting spacing and support details, a circuit unit in which the principles discussed with reference to Figs. 3, 4 and 5 are utilized. The magnet assembly comprises a set of identical coils 65 each of mean diameter D and of width, parallel to the longitudinal axis of the assembly, W. They are spaced apart a distance P between their central planes such that P 0.6 D The effect of an infinite length of assembly of these coils is simulated by means of the end plates 16 and l together with the'end coils 66 which are of the same mean diameter as the coils 65 but of smaller width w. The spacing between adjacent edges of neighbouring coils is d through the assembly. Were the end plates 10 and 16 of infinite extent and zero reluctance and the Width WZW/Z, then the end plates 10 and 16 would behave as magnetic mirrors providing virtual images of the system beyond the finite ends so that the finite number of coils would provide an axial field, between the end plates, identical with an infinite system of coils 65, mirror images of the coils 66 combining with their respective physical halves to provide a field contribution the same as that of the individual coils 65. The need for the end plates 16 and 10 to be of infinite extent is ob viated by providing a cylinder 67 of low, and ideally of zero, reluctance about the coil assembly between the end plates, apertures being provided for passage of the wave guides. Due to the finite reluctance of the iron path, the end coils 66 must have a greater width than in the theoretical system and in practice it is found that satisfactory results are obtained when w-0.75W. As stated previously, although the provision of a cylinder 67 is practicable, we find the tie-rods 18 of Fig. 1 sufficiently effective and more convenient.

, The arrangement of Fig. 6 differs in one rather important aspect from that of Fig. 1, in addition to the differences already noted in that a cylinder such as cylinder 38 of Fig. 1 cannot be allowed to project inside the coil system. Instead, for use with the same type of electron gun as in Fig. l, a member 60, as in Fig. 2, projects to the rear of the end plate 16, the end of the internal pole-piece member of the electron gun of the travelling- Wave tube being flush with the front surface of plate 16 as is indicated in Fig. 6 by the dotted lines showing the gun position.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

l. A circuit unit for providing magnetic focusing of an electron beam, for use with a travelling wave tube having an electron gun at one end for producing a beam, a collector electrode at the other end, and a wave transmission medium therebetween, all enclosed in an envelope, and provided with wave energy transfer coupling to said wave transmission medium of said tube, comprising an assembly of solenoids positioned on a common axis, supporting means for said solenoids provided with aligned tube receiving apertures coaxial with said solenoids, a wave energy transmission means mounted between two of said solenoids and provided with tube receiving apertures for energy transfer coupling with said wave transmission medium, a pair of centrally apertured end plates of ferromagnetic material mounted at opposite ends of said solenoid assembly, and a plurality of ferromagnetic clamping rods mounted around said solenoids to provide a low reluctance magnetic path between said end plates and to clamp said assembly together as aunitary structure.

2. A cir'cuitunit according to claim 1 in which the said wave transmission medium comprises a rectangular wave guide.

3. A circuit unit for providing magnetic focusing of an electron beam, for use with a travelling wave tube having an electron gun at one end for producing a beam, a collector electrode at the other end, and a wave transmission medium therebetween, all enclosed in an envelope, and provided with wave energy transfer coupling to said wave transmission medium of said tube, comprising an assembly of solenoids positioned on a common axis, supporting means for said solenoids provided with aligned tube receiving apertures coaxial with said solenoids, a wave energy transmission means mounted between two of said solenoids and provided with tube receiving apertures for energy transfer coupling with said wave transmission medium, a pair of centrally apertured end plates of ferro magnetic material mounted at opposite ends of said sole noid assembly, a plurality of ferromagnetic clamping rods mounted around said solenoids to provide a low reluctance magnetic path between said end plates and to clamp said assembly together as a unitary structure, a collar of insulating material having a bearing surface of restricted area on its inner face mounted in the aperture of one of said end plates providing means to support a travelling wave tube about the electron collector end thereof.

4. A circuit unit according to claim 3 wherein the travelling-wave tube is provided with a base flange at its electron gun end, further comprising a mounting bracket secured to the other said end plate external of said clamped assembly to provide a seating for said flange and means for locking the said flange in position against the said seating.

5. A circuit unit for providing magnetic focusing of an electron beam, for use with a travelling wave tube having an electron gun at one end for producing a beam, a collector electrode at the other end, and a wave transmission medium therebetween, all enclosed in an envelope, and provided with wave energy transfer coupling to said wave transmission medium of said tube, comprising an assembly of solenoids positioned on a common axis, supporting means for said solenoids provided with aligned tube receiving apertures coaxial with said solenoids, a Wave energy transmission means mounted between two of said solenoids and provided with tube receiving apertures for energy transfer coupling with said wave transmission medium a pair of centrally apertured end plates of ferromagnetic material mounted at opposite ends of said solenoid assembly, a plurality of ferromagnetic clamping rods mounted around said solenoids to provide a low reluctance magnetic path between said end plates and to clamp said assembly together as a unitary structure, wherein one of said supporting means comprises a pair of bushes of non-magnetic material positioned in opposite walls of the said wave guide about the apertures and a pair of circularly flanged solenoid supporting plates mounted one on each of the said bushes in contact with the wave guide providing means for supporting the adjacent ends of a pair of the said solenoids between which the said wave guide is positioned.

6. A circuit unit according to claim 5 further comprising a second wave guide each provided with said bushes and supporting plates, two spaced-apart solenoids intermediate the said wave guides and a cylinder of non-magnetic material supporting the ends of the said two solenoids remote from the respective wave guides, and a central cylindrical flange on the said collar spacing the said solenoids apart.

7. A circuit unit according to claim 6 further comprising a cylindrical coil former for the end solenoids of said assembly each having a circular flange projecting at one end beyond the end of the coil forming said solenoids and in which a circular recess is provided in the adjacent end plate to receive the said flange on the coil former providing support for the solenoids at the end of the assembly.

8. A circuit unit according to claim 6 further comprising a cylinder of ferromagnetic material projecting rearwards of the end plate of the magnet assembly at the electron gun end of the travelling-wave tube to surround the envelope of the tube at the base thereof.

9. A circuit unit for providing magnetic focusing of an electron beam, for use with a travelling wave tube having an electron gun at one end for producing a beam, a collector electrode at the other end, and a wave transmission medium therebetween, all enclosed in an envelope, and provided with wave energy transfer coupling to said wave transmission medium of said tube, comprising an assembly of solenoids positioned on a common axis, supporting means for said solenoids provided with aligned tube re- 8 ceiving apertures coaxial with said solenoids, a wave energy transmission means mounted between two of said solenoids and provided with tube receiving apertures for energy transfer coupling with said wave transmission medium, a pair of centrally apertured end plates of ferromagnetic material mounted at opposite ends of said solenoid assembly, a plurality of ferromagnetic clamping rods mounted around said solenoids to provide a low reluctance magnetic path between said end plates and to clamp said assembly together as a unitary structure, said assembly of solenoids comprising at least three solenoids substantially identical, except for the width of the two end solenoids parallel to said axis, mounted with equal spacings between adjacent solenoids, whereby the magnetic effect produced is proportional to said width, said two end solenoids being between one half and three quarters of the width of the remainder of the solenoids.

References Cited in the file of this patent UNITED STATES PATENTS 2,300,052 Lindenblad Oct. 27, 1942 2,305,884 Litton Dec. 22, 1942 2,602,148 Pierce July 1, 1952 2,619,611 Norton et a1. Nov. 25, 1952 2,629,066 Eitel et a1 Feb. 17, 1953 2,658,393 Woods Nov. 10, 1953 2,741,718 Wang Apr. 26, 1956 2,797,353 Molnar et al June 25, 1957 

